Today we’re going to look at how pressure sensors are used to measure level.
Wait, a level sensor that measures level by measuring pressure? How does that make any sense? If I wanted a pressure sensor, I would buy a pressure sensor! This is just confusing.
Hang on, hang on! Patience, young padawan. We have much to learn today.
So, two questions: A) how do pressure sensors measure level, and B) how do non-pressure sensors measure level? Since we’re more interested in the first, let’s quickly answer the second so we have more time to explore the first.
Non-pressure level sensors come in a few variations. There are floating sensors, such as magnetostrictive level sensors, which use an electrical signal to pinpoint the location of magnetic float on a vertical guide. There are ultrasonic and laser level sensors, which use sound or light waves to measure the distance from the sensor to a surface. And there are simple float switches, which can be arranged in an array to indicate various levels.
Each of these technologies has its benefits (e.g., non-contact for ultrasonic and laser; multiple floats for multiple liquids in a single vessel for magnetostrictive; etc.) and drawbacks (plain old complicated and imprecise for arrayed float switches).
Now pressure, to get all sciencey, is the measurement of a force applied over an area (i.e., 1 Pascal is defined as 1 Newton per square meter). And weight is a force (1 Newton is 1 kilogram-meter per second-squared). So weight measured across a surface is pressure.
*yawn* Thank you, high school physics class.
Using a pressure sensor to measure level is premised on water (any liquid, really, but we’ll use water for example) having a uniform weight at a given depth. In other words, a column of water of a specific height will always exert the same amount of pressure on a transducer. And because weight is a force, and pressure is the force applied over an area, in this case, the area of the transducer, we can take the pressure reading from the transducer and translate it into a level based on the specific gravity of the liquid. More on that later.
Programming Alert: More high school math ahead!
Pressure in still-ish water (code name: hydrostatic pressure) is calculated by multiplying the density of the water by gravity and the distance between the point in question and the surface of the water (P=ρgh). This equation is easily turned around to solve for said distance or depth (h=P/ρg). Lo and behold, since density and gravity are constant for water, the measurement of our pressure sensor converts quickly and accurately to a level measurement (2.31 is the magic number that turns PSI into feet).
Pretty slick, right?
So, while the vertical guide of a magnetostrictive sensor must be long enough to encompass both the minimum and maximum depth, and ultrasonic and laser sensors must be configured based on the distance from the sensor to the empty bottom of the vessel, a pressure sensor only needs to account for any depth below its placement level. Any water above the sensor is already accurately measured.
What’s that you say? Gravity isn’t a constant? Well, true, gravity does vary with elevation. But not so much that any one sensor needs to treat gravity as anything other than a constant.
What else? What about air pressure and specific gravity? Ok, ok. We can deal with those, too. Yes, changes in air pressure will change the absolute pressure. But absolute pressure is gauge pressure plus atmospheric pressure (Pabs = Patm + ρgh). So unless your sensor is designed to measure absolute pressure, changes in atmospheric pressure will not affect your level reading. Most submersible pressure gauges are vented, allowing the transducer to physically adjust to air pressure.
As for specific gravity, it’s a ratio of the density of one substance to the density of water. So if you’re monitoring water level, there’s no adjustment necessary. If you are monitoring a different fluid, an adjustment can be arranged with the manufacturer of your sensor to account for the difference. It is true that specific gravity is also affected by large swings in temperature. This, however, is easily fixed with temperature compensation internal to the pressure sensor.
Measuring levels with pressure is accurate, stable, and effective in non-pressurized tank environments. Pressure sensors themselves are less expensive than the purpose-built alternatives. And using pressure sensors is definitively easy and void of any mentionable complexity.
Additionally, there are a host of process fittings for practically any mounting need, even 3A compliance. Therefore, as a general rule, liquid level measurement with pressure sensors is a good way to go.
There are certain applications where using a pressure sensor to detect liquid level is particularly appropriate. Turbulence, foam, vapor layers, and physical obstructions are all common occurrences in the world of tanks and liquids, and can negatively affect other level sensors. Pressure sensors, however, are not affected by any of these and are a great alternative in such applications.
For pressurized tanks, using pressure sensors is still a viable option. However, it is a bit more complex and typically requires more than one sensor.
Feeling more comfortable with the idea of using a pressure sensor for level monitoring? Good! Still have questions? Let us know. You can call us, email us at sales@apgsensors.com, or engage our live chat for more information during business hours. You can also simply drop by our contact us page. We’d love to hear from you.
